1. Field of the Invention
The present invention relates to a fine-pattern forming process, and more particularly, to a process for manufacturing a semiconductor light emitting device including the fine patterns to improve light efficiency.
2. Description of the Related Art
Various semiconductor devices are being fabricated, such as light emitting diodes, laser diodes, photodiodes and transistors based on semiconductors.
For specific functions, the semiconductor device may be required to have fine patterns such as periodic/non-periodic patterns at a predetermined region. Such fine patterns may be formed by etching a semiconductor surface using a known etching process.
In the case of a nitride semiconductor light emitting device, light-extraction efficiency is limited due to a difference in a refractive index between the outside and a nitride semiconductor. In order to overcome this limitation, a fine-pattern structure may be formed in a surface of the nitride semiconductor light emitting device.
A photonic crystal structure having fine periodic grating patterns is being actively studied in order to improve luminance of a semiconductor light emitting device. Also, similar fine grating patterns are being adopted for a method for improving luminance by using a principle of surface Plasmon resonance.
However, an etching process used in this patterning process has limitations in forming fine patterns on a semiconductor surface. The limitations vary according to an etching method being used.
For example, dry etching such as reactive ion etching (RIE) and inductively coupled plasma reactive ion etching (ICP-RIE) can secure precise and reproducible patterns because it allows power control and has anisotropy. However, the dry etching has limitations in that properties of a semiconductor surface easily deteriorate due to a physical bombardment with ions or neutral atoms during the dry etching. Even if a thin film of a material, which is not p-type GaN, is deposited on a p-type GaN layer and then the thin film is patterned using dry etching, it is difficult to prevent damage to the p-type GaN layer placed at a portion where the thin film is removed.
A solid line of FIG. 1 represents a current-voltage (I-V) characteristic of a nitride semiconductor light emitting device purposely damaged by ICP-RIE using a halogen gas before an electrode is formed on a p-type GaN surface. A dotted line indicated by ‘X’ represents an I-V characteristic of a nitride semiconductor light emitting device before the damage occurs, which is different from an undamaged nitride LED indicated by ‘♦’. In the nitride semiconductor light emitting device damaged by the dry etching, a current begins to flow from a low voltage. However, this current is not the one that is generated by the normal carrier recombination but a leakage current that generates almost no light.
Therefore, research is ongoing on a method for recovering an original state of a crystal from damage caused by dry etching. However, because of nitrogen vacancy, a surface of a p-type GaN layer undergoes a change in its conductivity type into an n-type during an etching process. For this reason, using general post-processing cannot contribute to recovering the damaged crystal. The conductivity type conversion becomes a fatal defect in a p-n junction diode.
Unlike the dry etching, wet etching does not cause damage to a semiconductor surface such as p-type GaN. However, the wet etching also has limitations in that a specific plane (e.g., a c-plane) of a nitride single crystal is not etched almost at all, and precise patterning is difficult to achieve. Also, if an etching depth is excessive, a top end of a thin film is completely removed, and thus a photoresist layer serving as a mask is separated.